scholarly journals Immersion freezing of ice nucleating active protein complexes

2012 ◽  
Vol 12 (8) ◽  
pp. 21321-21353 ◽  
Author(s):  
S. Hartmann ◽  
S. Augustin ◽  
T. Clauss ◽  
J. Voigtländer ◽  
D. Niedermeier ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Nucleation Rate ◽  
Protein Complex ◽  
Protein Complexes ◽  
Ice Nucleation ◽  
Temperature Range ◽  
Immersion Freezing ◽  
New Perspective ◽  
Lower Temperature ◽  
Heterogeneous Ice Nucleation

Abstract. Biological particles, e.g. bacteria and their Ice Nucleating Active (INA) protein complexes, might play an important role for the ice formation in atmospheric mixed-phase clouds. Therefore, the immersion freezing behavior of INA protein complexes generated from a SnomaxTM solution/suspension was investigated as function of temperature in a range of −5 °C to −38 °C at the Leipzig Aerosol Cloud Interaction Simulator (LACIS). The immersion freezing of droplets containing small numbers of INA protein complexes occurs in a temperature range of −7 °C and −10 °C. The experiments performed in the lower temperature range, where all droplets freeze which contain at least one INA protein complex, are used to determine the average number of INA protein complexes present, assuming that the INA protein complexes are Poisson distributed over the droplet ensemble. Knowing the average number of INA protein complexes, the heterogeneous ice nucleation rate and rate coefficient of a single INA protein complex is determined by using the newly-developed CHESS model (stoCHastic model of idEntical poiSSon distributed ice nuclei). Therefore, we assume the ice nucleation process to be of stochastic nature, and a parameterization of the INA protein complex's nucleation rate. Analyzing the results of immersion freezing experiments from literature (SnomaxTM and Pseudomonas syringae bacteria), to results gained in this study, demonstrates that first, a similar temperature dependence of the heterogeneous ice nucleation rate for a single INA protein complex was found in all experiments, second, the shift of the ice fraction curves to higher temperatures can be explained consistently by a higher average number of INA protein complexes being present in the droplet ensemble, and finally the heterogeneous ice nucleation rate of one single INA protein complex might be also applicable for intact Pseudomonas syringae bacteria cells. The results obtained in this study allow a new perspective on the interpretation of immersion freezing experiments considering INA protein complexes and the derived simple parameterization of the heterogeneous ice nucleation rate can be used in cloud resolving models for studying the effect of bacteria induced ice nucleation.

scholarly journals Immersion freezing of ice nucleation active protein complexes

2013 ◽  
Vol 13 (11) ◽  
pp. 5751-5766 ◽  
Author(s):  
S. Hartmann ◽  
S. Augustin ◽  
T. Clauss ◽  
H. Wex ◽  
T. Šantl-Temkiv ◽  
...  
Keyword(s):  
Outer Membrane ◽  
Pseudomonas Syringae ◽  
Nucleation Rate ◽  
Protein Complex ◽  
Protein Complexes ◽  
Ice Nucleation ◽  
Minor Importance ◽  
Class Iii ◽  
Temperature Range ◽  
Immersion Freezing

Abstract. Utilising the Leipzig Aerosol Cloud Interaction Simulator (LACIS), the immersion freezing behaviour of droplet ensembles containing monodisperse particles, generated from a Snomax™ solution/suspension, was investigated. Thereto ice fractions were measured in the temperature range between −5 °C to −38 °C. Snomax™ is an industrial product applied for artificial snow production and contains Pseudomonas syringae} bacteria which have long been used as model organism for atmospheric relevant ice nucleation active (INA) bacteria. The ice nucleation activity of such bacteria is controlled by INA protein complexes in their outer membrane. In our experiments, ice fractions increased steeply in the temperature range from about −6 °C to about −10 °C and then levelled off at ice fractions smaller than one. The plateau implies that not all examined droplets contained an INA protein complex. Assuming the INA protein complexes to be Poisson distributed over the investigated droplet populations, we developed the CHESS model (stoCHastic modEl of similar and poiSSon distributed ice nuclei) which allows for the calculation of ice fractions as function of temperature and time for a given nucleation rate. Matching calculated and measured ice fractions, we determined and parameterised the nucleation rate of INA protein complexes exhibiting class III ice nucleation behaviour. Utilising the CHESS model, together with the determined nucleation rate, we compared predictions from the model to experimental data from the literature and found good agreement. We found that (a) the heterogeneous ice nucleation rate expression quantifying the ice nucleation behaviour of the INA protein complex is capable of describing the ice nucleation behaviour observed in various experiments for both, Snomax™ and P. syringae bacteria, (b) the ice nucleation rate, and its temperature dependence, seem to be very similar regardless of whether the INA protein complexes inducing ice nucleation are attached to the outer membrane of intact bacteria or membrane fragments, (c) the temperature range in which heterogeneous droplet freezing occurs, and the fraction of droplets being able to freeze, both depend on the actual number of INA protein complexes present in the droplet ensemble, and (d) possible artifacts suspected to occur in connection with the drop freezing method, i.e., the method frequently used by biologist for quantifying ice nucleation behaviour, are of minor importance, at least for substances such as P. syringae, which induce freezing at comparably high temperatures. The last statement implies that for single ice nucleation entities such as INA protein complexes, it is the number of entities present in the droplet population, and the entities' nucleation rate, which control the freezing behaviour of the droplet population. Quantities such as ice active surface site density are not suitable in this context. The results obtained in this study allow a different perspective on the quantification of the immersion freezing behaviour of bacterial ice nucleation.

scholarly journals Intercomparing different devices for the investigation of ice nucleating particles using Snomax<sup>®</sup> as test substance

2015 ◽  
Vol 15 (3) ◽  
pp. 1463-1485 ◽  
Author(s):  
H. Wex ◽  
S. Augustin-Bauditz ◽  
Y. Boose ◽  
C. Budke ◽  
J. Curtius ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Protein Complexes ◽  
Test Substance ◽  
Angle Distribution ◽  
Active Protein ◽  
Experimental Conditions ◽  
Slow Cooling ◽  
Suitable Material ◽  
Temperature Range ◽  
Immersion Freezing

Abstract. Seven different instruments and measurement methods were used to examine the immersion freezing of bacterial ice nuclei from Snomax® (hereafter Snomax), a product containing ice-active protein complexes from non-viable Pseudomonas syringae bacteria. The experimental conditions were kept as similar as possible for the different measurements. Of the participating instruments, some examined droplets which had been made from suspensions directly, and the others examined droplets activated on previously generated Snomax particles, with particle diameters of mostly a few hundred nanometers and up to a few micrometers in some cases. Data were obtained in the temperature range from −2 to −38 °C, and it was found that all ice-active protein complexes were already activated above −12 °C. Droplets with different Snomax mass concentrations covering 10 orders of magnitude were examined. Some instruments had very short ice nucleation times down to below 1 s, while others had comparably slow cooling rates around 1 K min−1. Displaying data from the different instruments in terms of numbers of ice-active protein complexes per dry mass of Snomax, nm, showed that within their uncertainty, the data agree well with each other as well as to previously reported literature results. Two parameterizations were taken from literature for a direct comparison to our results, and these were a time-dependent approach based on a contact angle distribution (Niedermeier et al., 2014) and a modification of the parameterization presented in Hartmann et al. (2013) representing a time-independent approach. The agreement between these and the measured data were good; i.e., they agreed within a temperature range of 0.6 K or equivalently a range in nm of a factor of 2. From the results presented herein, we propose that Snomax, at least when carefully shared and prepared, is a suitable material to test and compare different instruments for their accuracy of measuring immersion freezing.

scholarly journals The Leipzig Cloud Interaction Simulator (LACIS): operating principle and theoretical studies concerning homogeneous and heterogeneous ice nucleation

2010 ◽  
Vol 10 (11) ◽  
pp. 25577-25617
Author(s):  
S. Hartmann ◽  
D. Niedermeier ◽  
J. Voigtländer ◽  
T. Clauss ◽  
R. A. Shaw ◽  
...  
Keyword(s):  
Experimental Data ◽  
Numerical Model ◽  
Nucleation Rate ◽  
Rate Coefficient ◽  
Ice Nucleation ◽  
Freezing Behavior ◽  
Nucleation Theory ◽  
Dust Particles ◽  
Immersion Freezing ◽  
Heterogeneous Ice Nucleation

Abstract. At the Leipzig Cloud Interaction Simulator (LACIS) experiments investigating homogeneous and heterogeneous nucleation of ice (particularly immersion freezing in the latter case) have been carried out. Here both the physical LACIS setup and the numerical model developed to design experiments at LACIS and interpret their results are presented in detail. Combining results from the numerical model with experimental data, it was found that for the experimental parameter space considered, classical homogeneous ice nucleation theory is able to predict the freezing behavior of highly diluted ammonium sulfate solution droplets, while classical heterogeneous ice nucleation theory, together with the assumption of a constant contact angle, fails to predict the immersion freezing behavior of surrogate mineral dust particles (Arizona Test Dust, ATD). The main reason for this failure is the compared to experimental data apparently overly strong temperature dependence of the nucleation rate coefficient. Assuming, in the numerical model, Classical Nucleation Theory (CNT) for homogeneous ice nucleation and a CNT-based parameterization for the nucleation rate coefficient in the immersion freezing mode, recently published by our group, it was found that even for a relatively effective ice nucleating agent such as pure ATD, there is a temperature range where homogeneous ice nucleation is dominant. The main explanation is the apparently different temperature dependencies of the two freezing mechanisms. Finally, reviewing the assumptions made during the derivation of the parameterization, it was found that the assumption of constant temperature during ice nucleation and the chosen nucleation time were highly justified, underlining the applicability of both the method to determine the fitting coefficients in the parameterization equation, and the validity of the parameterization concept itself.

scholarly journals Intercomparing different devices for the investigation of ice nucleating particles using Snomax<sup>®</sup> as test substance

2014 ◽  
Vol 14 (16) ◽  
pp. 22321-22384 ◽  
Author(s):  
H. Wex ◽  
S. Augustin-Bauditz ◽  
Y. Boose ◽  
C. Budke ◽  
J. Curtius ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Protein Complexes ◽  
Test Substance ◽  
Angle Distribution ◽  
Active Protein ◽  
Experimental Conditions ◽  
Slow Cooling ◽  
Suitable Material ◽  
Temperature Range ◽  
Immersion Freezing

Abstract. Seven different instruments and measurement methods were used to examine the immersion freezing of bacterial ice nuclei from Snomax® (hereafter Snomax), a product containing ice active protein complexes from non-viable Pseudomonas syringae bacteria. The experimental conditions were kept as similar as possible for the different measurements. Of the participating instruments, some examined droplets which had been made from suspensions directly, and the others examined droplets activated on previously generated Snomax particles, with particle diameters of mostly a few hundred nanometers and up to a few micrometers in some cases. Data were obtained in the temperature range from −2 to −38 °C, and it was found that all ice active protein complexes were already activated above −12 °C. Droplets with different Snomax mass concentrations covering 10 orders of magnitude were examined. Some instruments had very short ice nucleation times down to below 1 s, while others had comparably slow cooling rates around 1 K min−1. Displaying data from the different instruments in terms of numbers of ice active protein complexes per dry mass of Snomax, nm, showed that within their uncertainty the data agree well with each other as well as to previously reported literature results. Two parameterizations were taken from literature for a direct comparison to our results, and these were a time dependent approach based on a contact angle distribution Niedermeier et al. (2014) and a modification of the parameterization presented in Hartmann et~al.~(2013) representing a time independent approach. The agreement between these and the measured data were good, i.e. they agreed within a temperature range of 0.6 K or equivalently a range in nm of a factor of 2. From the results presented herein, we propose that Snomax, at least when carefully shared and prepared, is a suitable material to test and compare different instruments for their accuracy of measuring immersion freezing.

scholarly journals Homogeneous and heterogeneous ice nucleation at LACIS: operating principle and theoretical studies

2011 ◽  
Vol 11 (4) ◽  
pp. 1753-1767 ◽  
Author(s):  
S. Hartmann ◽  
D. Niedermeier ◽  
J. Voigtländer ◽  
T. Clauss ◽  
R. A. Shaw ◽  
...  
Keyword(s):  
Experimental Data ◽  
Numerical Model ◽  
Nucleation Rate ◽  
Rate Coefficient ◽  
Ice Nucleation ◽  
Freezing Behavior ◽  
Nucleation Theory ◽  
Dust Particles ◽  
Immersion Freezing ◽  
Heterogeneous Ice Nucleation

Abstract. At the Leipzig Aerosol Cloud Interaction Simulator (LACIS) experiments investigating homogeneous and heterogeneous nucleation of ice (particularly immersion freezing in the latter case) have been carried out. Here both the physical LACIS setup and the numerical model developed to design experiments at LACIS and interpret their results are presented in detail. Combining results from the numerical model with experimental data, it was found that for the experimental parameter space considered, classical homogeneous ice nucleation theory is able to predict the freezing behavior of highly diluted ammonium sulfate solution droplets, while classical heterogeneous ice nucleation theory, together with the assumption of a constant contact angle, fails to predict the immersion freezing behavior of surrogate mineral dust particles (Arizona Test Dust, ATD). The main reason for this failure is the compared to experimental data apparently overly strong temperature dependence of the nucleation rate coefficient. Assuming, in the numerical model, Classical Nucleation Theory (CNT) for homogeneous ice nucleation and a CNT-based parameterization for the nucleation rate coefficient in the immersion freezing mode, recently published by our group, it was found that even for a relatively effective ice nucleating agent such as pure ATD, there is a temperature range where homogeneous ice nucleation is dominant. The main explanation is the apparently different temperature dependencies of the two freezing mechanisms. Finally, reviewing the assumptions made during the derivation of the CNT-based parameterization for immersion freezing, it was found that the assumption of constant temperature during ice nucleation and the chosen ice nucleation time were justified, underlining the applicability of the method to determine the fitting coefficients in the parameterization equation.

scholarly journals Heterogeneous ice nucleation activity of bacteria: new laboratory experiments at simulated cloud conditions

2008 ◽  
Vol 5 (5) ◽  
pp. 1425-1435 ◽  
Author(s):  
O. Möhler ◽  
D. G. Georgakopoulos ◽  
C. E. Morris ◽  
S. Benz ◽  
V. Ebert ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Cloud Condensation Nuclei ◽  
Bacterial Species ◽  
Ice Nucleation ◽  
Cloud Chamber ◽  
Bacterial Cells ◽  
Active Fraction ◽  
Temperature Range ◽  
Spray Droplets ◽  
Immersion Freezing

Abstract. The ice nucleation activities of five different Pseudomonas syringae, Pseudomonas viridiflava and Erwinia herbicola bacterial species and of Snomax™ were investigated in the temperature range between −5 and −15°C. Water suspensions of these bacteria were directly sprayed into the cloud chamber of the AIDA facility of Forschungszentrum Karlsruhe at a temperature of −5.7°C. At this temperature, about 1% of the Snomax™ cells induced immersion freezing of the spray droplets before the droplets evaporated in the cloud chamber. The living cells didn't induce any detectable immersion freezing in the spray droplets at −5.7°C. After evaporation of the spray droplets the bacterial cells remained as aerosol particles in the cloud chamber and were exposed to typical cloud formation conditions in experiments with expansion cooling to about −11°C. During these experiments, the bacterial cells first acted as cloud condensation nuclei to form cloud droplets. Then, only a minor fraction of the cells acted as heterogeneous ice nuclei either in the condensation or the immersion mode. The results indicate that the bacteria investigated in the present study are mainly ice active in the temperature range between −7 and −11°C with an ice nucleation (IN) active fraction of the order of 10−4. In agreement to previous literature results, the ice nucleation efficiency of Snomax™ cells was much larger with an IN active fraction of 0.2 at temperatures around −8°C.

scholarly journals Simulating the influence of primary biological aerosol particles on clouds by heterogeneous ice nucleation

2018 ◽  
Vol 18 (20) ◽  
pp. 15437-15450 ◽  
Author(s):  
Matthias Hummel ◽  
Corinna Hoose ◽  
Bernhard Pummer ◽  
Caroline Schaupp ◽  
Janine Fröhlich-Nowoisky ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Aerosol Particles ◽  
Fungal Spores ◽  
Ice Nucleation ◽  
Ice Crystals ◽  
Atmospheric Measurements ◽  
Cloud Ice ◽  
Biological Aerosol Particles ◽  
Ice Phase ◽  
Heterogeneous Ice Nucleation

Abstract. Primary ice formation, which is an important process for mixed-phase clouds with an impact on their lifetime, radiative balance, and hence the climate, strongly depends on the availability of ice-nucleating particles (INPs). Supercooled droplets within these clouds remain liquid until an INP immersed in or colliding with the droplet reaches its activation temperature. Only a few aerosol particles are acting as INPs and the freezing efficiency varies among them. Thus, the fraction of supercooled water in the cloud depends on the specific properties and concentrations of the INPs. Primary biological aerosol particles (PBAPs) have been identified as very efficient INPs at high subzero temperatures, but their very low atmospheric concentrations make it difficult to quantify their impact on clouds. Here we use the regional atmospheric model COSMO–ART to simulate the heterogeneous ice nucleation by PBAPs during a 1-week case study on a domain covering Europe. We focus on three highly ice-nucleation-active PBAP species, Pseudomonas syringae bacteria cells and spores from the fungi Cladosporium sp. and Mortierella alpina. PBAP emissions are parameterized in order to represent the entirety of bacteria and fungal spores in the atmosphere. Thus, only parts of the simulated PBAPs are assumed to act as INPs. The ice nucleation parameterizations are specific for the three selected species and are based on a deterministic approach. The PBAP concentrations simulated in this study are within the range of previously reported results from other modeling studies and atmospheric measurements. Two regimes of PBAP INP concentrations are identified: a temperature-limited and a PBAP-limited regime, which occur at temperatures above and below a maximal concentration at around −10 ∘C, respectively. In an ensemble of control and disturbed simulations, the change in the average ice crystal concentration by biological INPs is not statistically significant, suggesting that PBAPs have no significant influence on the average state of the cloud ice phase. However, if the cloud top temperature is below −15 ∘C, PBAP can influence the cloud ice phase and produce ice crystals in the absence of other INPs. Nevertheless, the number of produced ice crystals is very low and it has no influence on the modeled number of cloud droplets and hence the cloud structure.

scholarly journals A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot

2017 ◽  
Vol 74 (3) ◽  
pp. 699-717 ◽  
Author(s):  
Romy Ullrich ◽  
Corinna Hoose ◽  
Ottmar Möhler ◽  
Monika Niemand ◽  
Robert Wagner ◽  
...  
Keyword(s):  
Active Surface ◽  
Ice Nucleation ◽  
Nucleation Theory ◽  
Classical Nucleation Theory ◽  
Negative Slope ◽  
Organic Carbon Content ◽  
Desert Dust ◽  
Soot Aerosol ◽  
Immersion Freezing ◽  
Heterogeneous Ice Nucleation

Abstract Based on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach. The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechanism. The deposition nucleation measured for soot at temperatures below about 240 K also follows u-shaped isolines with a shift toward higher Si for soot with higher organic carbon content. For immersion freezing of soot aerosol, only upper limits for nS were determined and used to rescale an existing parameterization line. The new parameterization framework is compared to a CNT-based parameterization and an empirical framework as used in models. The comparison shows large differences in shape and magnitude of the nS isolines especially for deposition nucleation. For the application in models, implementation of this new framework is simple compared to that of other expressions.

scholarly journals Simulating the Influence of Primary Biological Aerosol Particles on Clouds by Heterogeneous Ice Nucleation

2018 ◽  
Author(s):  
Matthias Hummel ◽  
Corinna Hoose ◽  
Bernhard Pummer ◽  
Caroline E. Schaupp ◽  
Janine Fröhlich-Nowoisky ◽  
...  
Keyword(s):  
Pseudomonas Syringae ◽  
Aerosol Particles ◽  
Fungal Spores ◽  
Ice Nucleation ◽  
Ice Crystals ◽  
Atmospheric Measurements ◽  
Cloud Ice ◽  
Biological Aerosol Particles ◽  
Ice Phase ◽  
Heterogeneous Ice Nucleation

Abstract. Primary ice formation, which is an important process for mixed-phase clouds with impact on their lifetime, radiative balance and hence the climate, strongly depends on the availability of ice nucleating particles (INPs). Supercooled droplets within these clouds remain liquid until an INP immersed in or colliding with to the droplet gets reaches its activation temperature. Only a few aerosol particles are acting as INPs and the freezing efficiency varies among them. Thus, the fraction of supercooled water in the cloud depends on the specific properties and concentrations of the INPs. Primary biological aerosol particles (PBAPs) have been identified as very efficient INPs at high subzero temperatures, but their very low atmospheric concentrations make it difficult to quantify their impact on clouds. Here we use the regional atmospheric model COSMO-ART to simulate the heterogeneous ice nucleation by PBAPs during a 1-week case study on a domain covering Europe. We focus on three highly ice nucleation active PBAP species, Pseudomonas syringae bacteria cells and spores from the fungi Cladosporium sp. and Mortierella alpina. PBAP emissions are parameterized in order to represent the entirety of bacteria and fungal spores in the atmosphere. Thus, only parts of the simulated PBAP are assumed to act as INP. The ice nucleation parameterizations are specific for the three selected species and are based on a deterministic approach. The PBAP concentrations simulated in this study are within the range of previously reported results from other modelling studies and atmospheric measurements. Two regimes of PBAP INP concentrations are identified: a temperature-limited and a PBAP-limited regime, which occur at temperatures above and below a maximal concentration at around −10 °C, respectively. In an ensemble of control and disturbed simulations, the change in the average ice crystal concentration by biological INPs is not statistically 1significant, suggesting that PBAP have no significant influence on the average state of the cloud ice phase. However, if the cloud top temperature is below −15 °C, PBAP can influence the cloud ice phase and produce ice crystals in the absence of other INPs. Nevertheless, the number of produced ice crystals is very low and it has no influence on the modelled number of cloud droplets and hence the cloud structure.

scholarly journals A new multi-component heterogeneous ice nucleation model and its application to Snomax bacterial particles and a Snomax-illte mineral particle mixture

2017 ◽  
Author(s):  
Hassan Beydoun ◽  
Michael Polen ◽  
Ryan C. Sullivan
Keyword(s):  
Cloud Microphysics ◽  
Ice Nucleation ◽  
Freezing Behavior ◽  
Dust Particles ◽  
Mineral Particle ◽  
Critical Surface ◽  
Wide Range ◽  
Nucleation Model ◽  
Immersion Freezing ◽  
Heterogeneous Ice Nucleation

Abstract. Some biological particles, such as Snomax, are very active ice nucleating particles, inducing heterogeneous freezing in supercooled water at temperatures above −15° C and up to −2° C. Despite their exceptional freezing abilities, large uncertainties remain regarding the atmospheric abundance of biological ice nucleating particles, and their contribution to atmospheric ice nucleation. It has been suggested that small biological ice nucleating macromolecules or fragments can be carried on the surfaces of dust and other atmospheric particles. This could combine the atmospheric abundance of dust particles with the ice nucleating strength of biological material to create strongly enhanced and abundant ice nucleating surfaces in the atmosphere, with significant implications for the budget and distribution of atmospheric ice nucleating particles, and their consequent effects on cloud microphysics and mixed-phase clouds. The new critical surface area “g” framework that was developed by Beydoun et al. (2016), is extended to produce a heterogeneous ice nucleation mixing model that can predict the freezing behavior of multi-component particle surfaces immersed in droplets. The model successfully predicts the immersion freezing properties of droplets containing Snomax bacterial particles across a mass concentration range of seven orders of magnitude, by treating Snomax as comprised of two distinct distributions of heterogeneous ice nucleating activity. Furthermore, the model successfully predicts the immersion freezing behavior of a low concentration mixture of Snomax and illite mineral particles, a proxy for the biological-dust mixtures observed in atmospheric aerosols. It is shown that even at very low Snomax concentrations in the mixture, droplet freezing at higher temperatures is still determined solely by the second less active and more abundant distribution of heterogeneous ice nucleating activity of Snomax, while freezing at lower temperatures is determined solely by the heterogeneous ice nucleating activity of pure illite. This demonstrates that in this proxy system, biological ice nucleating particles do not compromise their ice nucleating activity upon mixing with dust and no new range of intermediary freezing temperatures associated with the mixture of ice nucleating particles of differing activities is produced. The study is the first to directly examine the freezing behavior of a mixture of Snomax and illite and presents the first multi-component ice nucleation model experimentally evaluated using a wide range of ice nucleating particle concentration mixtures in droplets.

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